S40 Knock Sensor Issue

I had a similar issue with my S40. The symptoms you describe (check engine, poor performance, uneven idling) match what I experienced when my knock sensor was going bad. Have you checked for any visible damage to the sensor itself, or the surrounding wires? Is the ignition timing correct?

Good call on the sensor check! Aside from a little grime, the sensor looks fine. I did spray some contact cleaner on the plug, maybe that threw the ignition timing out of whack? Is there a good way to test the sensor's output directly, without, you know, detonating something?

While cleaning the plug might help temporarily, the underlying problem will likely persist and you will have poor performance again. In my case, I eventually had to take it to a workshop for a proper diagnosis and replacement. It's probably worth getting it checked, to avoid damaging the engine due to incorrect ignition timing.

Thanks for the reality check! Dropped it off at the shop this morning. Turns out it WAS the knock sensor connection, just like I suspected, but the corrosion was hidden inside the plug. They cleaned everything up and replaced the connector, and now she's purring like a kitten again. Final bill was 95€, so not too bad. Thanks for the help!